Solid-state image sensors (also known as “image sensors,” “imagers,” or “solid-state imagers”) have broad applications in many areas including commercial, consumer, industrial, medical, defense and scientific fields. Solid-state image sensors convert a received image such as from an object into a signal indicative of the received image. Examples of solid-state image sensors including charge coupled devices (“CCD”), photodiode arrays, charge injection devices (“CID”), hybrid focal plane arrays and complementary metal oxide semiconductor (“CMOS”) imaging devices.
Solid-state image sensors are fabricated from semiconductor materials (such as silicon or gallium arsenide) and include imaging arrays of light detecting (i.e., photosensitive) elements (also known as photodetectors) interconnected to generate analog signals representative of an image illuminating the device. These imaging arrays are typically formed from rows and columns of photodetectors (such as photodiodes, photoconductors, photocapacitors or photogates), each of which generate photo-charges. The photo-charges are the result of photons striking the surface of the semiconductor material of the photodetector, which generate free charge carriers (electron-hole pairs) in an amount linearly proportional to the incident photon radiation.
Each photodetector in the imaging array receives a portion of the light reflected from the object received at the solid-state image sensor. Each portion is known as a picture element or “pixel.” Each individual pixel provides an output signal corresponding to the radiation intensity falling upon its detecting area (also known as the photosensitive or detector area) defined by the physical dimensions of the photodetector. The photo-charges from each pixel are converted to a signal (charge signal) or an electrical potential representative of the energy level reflected from a respective portion of the object. The resulting signal or potential is read and processed by video processing circuitry to create an electrical representation of the image. This signal may be utilized, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image.
CCDs are commonly utilized as solid-state image sensors. However, CMOS technology has made significant strides in competing with CCD technology as the solid-state image sensor of choice for use in various applications such as stand-alone digital cameras and digital cameras embedded in other imaging devices (e.g., cellular phones and personal digital assistants). The principal advantages of CMOS technology are lower power consumption, higher levels of system integration that enable the creation of “camera-on-a-chip” capabilities, the ability to support very high data rates and the ease of manufacturing through the utilization of standard CMOS wafer fabrication facilities.
In video systems, CMOS technology is capable of higher frame rates than CCD technology at the same or lower levels of circuit noise because many of the elements can be designed to operate in parallel. In CCD circuits, a single amplifier transforms the received charge to voltage and supports the total data rate of the solid-state image sensor's frame rate. In CCD solid-state image sensors, the amplifier noise generally becomes dominant when 30 frames per second (FPS) is employed for image sizes over several hundred thousand pixels.
CMOS solid-state image sensors, on the other hand, utilize multiple amplifiers that allow a longer settling time between applications and higher frame rate while maintaining excellent noise rejection. In addition, CMOS solid-state image sensors may easily be equipped with a precision analog-to-digital (“A/D”) converter on the solid-state image sensor chip.
In many imaging applications, it is often desirable to take a snap shot of a video image (i.e., to obtain a still image). Unfortunately, because video images are not generally of the highest quality, the snap shot of the still image will also not be of the highest quality. Such snap shots are especially inferior when compared with typical still images generated in accordance with any one of a number of still image techniques or standards generally known in the art. Typically, these higher quality still images are generated utilizing specialized image generation software.
Generally, conventional CCD solid-state imager sensors provide snap shot capability through an interline transfer approach. In the interline transfer approach, when a short exposure is required to freeze the action, the charge is transferred from the light collection junction to a junction shielded from light. The information regarding the light level is then stored on a storage node in the dark until the frame can be read. This method typically reduces motion blur and allows motion to be frozen even when the time to read the entire frame is much longer than the integration time for the exposure.
Conventional CMOS solid-state image sensors have also attempted to solve this snap shot capability problem by incorporating a storage node in the cell. However, this storage node must allow the transfer of the charge from the light collection nodes to the storage nodes, which requires an additional transistor in the cell. Such active pixel sensors are often termed four-transistor cells to distinguish them from the three-transistor active pixel sensor in CMOS solid-state image sensors. Typically, the transfer of charge from the light collection node to the storage node introduces additional reset or kT/C noise unless a very specialized field effect transistor (“FET”) design is used. Additionally, cross talk may cause the storage node to continue to respond to light at 10% to 20% of the response of the lighted node. Moreover, the area required to implement the storage node also reduces the area available for light collection. Generally, for small pitch solid-state image sensor cells, the installation of a storage node reduces the available area for light collection by about 30% to 50%. The combined effects of less light collection area, transfer kT/C noise and cross talk may cause the four-transistor cell to have a signal-to-noise performance that is about ⅓ that of a conventional three-transistor cell of the same pitch. Therefore, there is a need for a high performance solid-state image sensor that solves the snap shot capability problem.